1. Field of the Invention
This invention is directed to a method of obtaining multilayer dopant distributions, including junctions, in semiconductor materials.
2. Description of the Prior Art
In semiconductor processing, a typical method of obtaining a desired distribution of dopants is by diffusion through the surface of a semiconductor body, typically using an oxide mask. Minimum device dimensions are typically related to the smallest lithographic features on the mask. Dopant impurities can also be introduced into a semiconductor body by means of ion implantation. Both surface doped regions and buried regions may be obtained by this technique. Typically, to activate the dopant impurities and to remove damage to the crystal following the ion implantation, the semiconductor is annealed at an elevated temperature.
Recently, the use of laser annealing has been widely studied. In this technique, a laser beam is used to raise the temperature of the semiconductor and thereby remove defects resulting from the ion implantation. Both solid state laser annealing and melting by means of a laser have been utilized to remove defects. Selective radiation by means of a laser can also be used to form devices in the doped layer. For example, a semiconductor body having an initial p-doping may be implanted with n-type dopant ions. A laser may be directed at the body to melt through the ion-implanted region into the underlying single crystal region. Upon resolidification of the melt, a p-n junction is typically obtained.
Laser radiation has also been used to form silicide surface layers on silicon, typically for use as ohmic contacts and for Schottky barrier devices. In this method, a metal, typically a transition metal such as tantalum, niobium, etc., is coated onto the surface of a silicon wafer or else introduced into the surface by means of ion implantation, and laser radiation is used to melt the surface, thereby forming a silicide region in the wafer.
Selective laser irradiation has the advantage that only a small portion of a semiconductor wafer need be heated at any given moment, with the remainder of the wafer remaining relatively cool. In this method, undesired diffusion of dopants in the wafer is avoided. An overview of the current state of the art of laser processing is given in Laser-Solid Interactions And Laser Processing--1978, AIP Conference Processings, No. 50, S. D. Ferris, H. J. Leamy, J. M. Poate, editors, published by the American Institute Of Physics, New York (1979).
In addition to laser melting, the use of electron beams and ion beams is known for melting, although lasers are typically presently preferred, due to the ease of implementation. In addition, xenon flash lamps have been used for processing in the solid phase, and work has been directed towards obtaining melting of semiconductor material by means of flash lamps.